With the proliferation of audio playback devices in use today, demand is rising for improved quality from these devices. One factor that can significantly affect the perceived audio quality of a playback device is the presence and audibility of background or environmental noise. This problem exists for most, if not all, classes of playback devices, whether they employ a built-in or detached speaker or speakers, transmit the audio signal wirelessly to a single earpiece (for example, Bluetooth™ headsets), or transmit the audio signal to stereo headphones, either wirelessly or via a standard or proprietary wired connection. Many products currently on the market offer active noise cancellation (ANC) technology which attempts to acoustically cancel some of the background or environmental noise in the electroacoustic channel at the entrance to the ear canal. The acoustic signal at the entrance to the ear canal is acquired through a small microphone placed in close proximity to the speaker (driver) such that said microphone is capable of sensing the signal played out through the driver, as well as the ambient environmental noise. The amount and bandwidth of noise cancellation varies significantly depending on the ANC technique used. However, due to fundamental limitations of existing ANC techniques, they generally do not provide significant noise reduction for frequencies above about 1 kHz, and may even, in some cases, increase noise levels of frequencies above 1 kHz.
Another technology currently available for reducing the effects of noisy ambient environments is dynamic noise compensation (DNC). In this technology, the spectral characteristics of the ambient noise from the environment are analyzed, and the playback level of the audio signal is selectively adjusted in response. In spectral regions in which the background noise is not deemed distracting, the audio signal is left largely unmodified. However, in spectral regions in which the background noise level is high enough to negatively affect the perceived quality or audibility of the audio signal, a level adjustment is made to the audio signal to improve the audio quality for the listener.
A third process for improving fidelity to the original signal is the use of equalization, which operates to correct the frequency response of the electroacoustic channel using inverse filtering techniques referred to as adaptive equalization (AEQ).